The invention generally relates to an apparatus for, and a method of, raster scanning of an image. In particular, it relates to the provision of timing, synchronisation and/or locating information during the electro-optical scanning of an indicia, in a direction perpendicular to the scanning direction. The invention is particularly although not exclusively useful in the type of raster scanning apparatus known as a laser camera.
Bar code scanners normally read bar code symbols by very rapidly and repeatedly scanning a light beam, for example from a laser source, across the symbol or bar code symbol structure to be deciphered. Reflected light from the light and dark areas of the bar code is received by the scanner, with the time delays between the resultant received pulses of light being used as the basis of a calculation to determine the spacial relationship between light and dark areas which are present in the bar code. Because of natural variations in the scanning speed (both between scans and within a single scan), and other unquantifiable variables such as movement of the scanner in the user's hand during the scanning process, some means of location and synchronisation is required to ensure that the received series of light pulses can reliably be converted into the proper spacial relationship. This locating and timing or synchronisation information is normally built into the bar code. This can be done in many ways, but one convenient possibility is to include a series of special locating, timing or synchronisation lines within the bar code symbol structure, spaced apart by a known distance. Receipt of signals from these lines enables the scanner to correct for any non-uniformities in the scanning speed.
Because of their need for locating and timing information, bar code scanners have not widely been used to decode indicia such as signatures which intrinsically contain no locating or timing information. The problem of scanning signatures has been addressed by U.S. Pat. No. 5,138,140, which is in common ownership with the present application. That patent teaches that one can reliably scan a signature with a bar code scanner, provided that the label on which the signature is written also carries a multi-row preamble identification code in front of the signature, and a corresponding postamble code after the signature. In practice, labels are pre-printed with preamble and postamble codes, and the signatory is asked to sign in the space between them. The codes and the signature are then scanned. The timing information provided by the preamble and postamble codes enables the reflected light pulses from the signature to be accurately converted into a digitized spacial reconstruction of the signature, held in computer memory.
Although effective to achieve its stated purpose, the teaching of the prior art does require the use of relatively expensive pre-printed labels. This not only adds to the cost, but limits the flexibility of the system since to be effectively scanned a signature has to be written precisely in the specified area between the preamble and postamble codes. Signatures of different sizes are not easily catered for, neither is it easy to generalize the system to allow the scanner to read many types of images, of different sizes and orientations.
A further particular difficulty arises when one is scanning an image or other indicia by means of a raster scanning pattern. If one takes the "X" direction to be the scanning direction, and the "Y" direction to be the perpendicular direction, it is found in practice that it is more difficult to calibrate the scan in the Y direction that it is in the X direction. The reason is that the scanning in the X direction is relatively rapid, and hence a large number of scans can be completed, thereby providing a substantial amount of feed-back information; movement in the Y direction, on the other hand, is slow and the amount of feed-back information is correspondingly limited. Knowledge of the instantaneous Y position of the scanning beam can be crucial in some circumstances, particularly when one is tying to scan an indicia such as a matrix code symbol or a stacked bar code symbol. It is impossible properly to decode such symbols unless one can be certain of which particular row is being read at any given instant. If Y positional information is not available, one may be reading a different part of the symbol from that which one is expecting. Furthermore, without Y positional information one cannot tell reliably whether the raster scanning pattern is skewed relatively to the symbol, so that a single scan line crosses several rows of the indicia.